Why the output speed of a fluid coupling is always lower than the input speed?

To enable the fluid to flow from impeller to rotor it is essential that there is a difference in 'Head' between the two and thus it is essential that there is a difference in speed known as slip, between the two. Slip is an important and inherent characteristic of a fluid coupling resulting in several advantages. As the slip increases, more and more fluid can be transferred from the impeller to the rotor and more torque is transmitted.

How does the operating fluid quantity in the fluid coupling affect the torque transmission behavior?

By varying the quantity of the oil in the fluid coupling, the normal torque transmitting capacity can be varied. The maximum torque or limiting torque of the fluid coupling can also be set to a predetermined value by adjusting the oil filling.

Characteristics of operating fluid affects the transmission behavior of a fluid coupling. The higher the density of the operating fluid, the better the transmission capacity. The higher the viscosity of the operating fluid, the more unfavorable the transmission behavior. The viscosity Index and flash points of operating fluid are also important. It must be ensured that the operating fluid is compatible with coupling components and their materials and operating conditions.

It is observed that the fluid coupling temperature rises during operation. Is this normal?

A fluid coupling has operating slip, therefore loss is converted into heat. The temperature rise is normal. If temperature rises above specified values then the cause must be established and rectified.

What is the purpose of Thermal Protection on constant fill fluid couplings?

Thermal protection on a fluid coupling provides safety to the fluid coupling and directly to the driver and driven machine.

Fusible plug is the most common thermal protection. If the temperature of fluid in the coupling increases for any reason (which includes overloads) then the fusible metal in plug melts & all the fluid in the coupling drains out thus stopping power transmission and over loading the prime mover.

Additional thermal tripping systems include:

Type MTP with an ejector pin & limit switch to stop the motor in case of temperature rise over preset limit.

When do we select fluid coupling with constant fill and when do we select a variable speed scoop controlled coupling?

Normally it depends very much on the application and design parameter of the drive system. Soft start, starting duty, controlled acceleration, multi drive systems etc are some of the parameters considered for use of constant fill fluid couplings.

A certain quantity of oil is stored in the delay fill chamber while the coupling is stationary. When the motor is started there is less oil in the working circuit which results in faster motor acceleration and lower starting torque. After some period the oil in the delay fill chamber drains into the working circuit resulting in smooth acceleration of driven machine and better efficiency during operation.

All exposed rotating parts must be guarded against contact. The guard must also be designed to catch any oil released by the coupling in the event that the fusible plug blows off. The guard must be so designed so that it does not impair the ventilation of the fluid coupling.

The first fill of Oil should be drained after the first 400 working hours. This oil can be filtered and reused or fresh oil may be filled. After 1st fill change, oil can be used for next 15000 working hours. Oil level should be checked periodically & if needed should be topped up.

The serial number of a fluid coupling is very important for all queries and spare parts orders. It helps to clearly identify the coupling. For constant-fill fluid couplings, the serial number and the coupling type are punched in the outer wheel, either at the outer periphery or close to it.

For variable speed fluid couplings, the serial number is shown on the identification name plate and also punched on the top side of the housing.